resourceFetcher

Fetches the resources that the book names by an absolute https URL instead of a file in its container (EPUB 3.3, 3.6, #38). kitepdf-net ships one on a Ktor client. Null fetches nothing: a remote image takes its manifest fallback, else keeps the room its markup gives, and a remote font takes the next source of its rule. See EpubDocument.fetchRemoteResources for when the bytes count.